Flyback-derived chargers and similar adapters (generically termed “converters” hereafter) often have a cable between a voltage source and a load. This cable drops some voltage as the load current increases, unless there is compensation by a source controller integrated circuit (IC). Primary-side controllers using optically-coupled feedback from the secondary side of a regulator cannot incorporate cable compensation, so this function is usually accomplished external to the regulator. Opto-coupled feedback is commonly known as galvanic isolation and prevents a continuous electrically conductive path between output and input. Although the optical feedback path is used to regulate the voltage delivered to the regulator output, detecting a voltage drop across the cable due to its wire resistance cannot be accomplished using common feedback loop mechanisms. Moreover, a primary-side controller is generally incapable of over-riding the reference voltage of the secondary-side regulator without some elaborate additional circuitry with its own isolation.
Controllers located on the primary side often employ Primary-Side Regulation (PSR) techniques for adjusting the converter's regulation reference based on primary-side current information indicative of the load current. However, PSR is subject to transient response limitations that may be inadequate for certain application performance requirements. Instead, a low-cost secondary-side shunt regulator is used to generate an error signal which is optically coupled to the primary-side controller which controls the power conversion based on the feedback signal level.